Permanent magnet rotor

ABSTRACT

A permanent magnet rotor having a unitary aluminum casting surrounding magnets mounted on the shaft which has favorable heat transfer, performance, and cost characteristics, and which may be constructed by the accompanying method. Spacers and rings surrounding the permanent magnets are eliminated by the aluminum casting. Pockets for the permanent magnets are cast into the aluminum. A non-magnetic outer sleeve overlies the shaft, magnets and aluminum layer.

This is a division of application Ser. No. 515,331 filed July 19, 1983.

BACKGROUND OF THE INVENTION

Generators for use in aerospace applications are high output, high speeddevices with critical weight and volume restrictions. Such devicestypically have permanent magnet rotors, which have a plurality of rareearth permanent magnet elements located on a shaft of magnetizablematerial. The shaft has a number of flat machined faces about itsperiphery, onto which the rare earth permanent magnet elements aremounted.

Spacers of aluminum or some other non-magnetizable material are locatedaround the periphery of the shaft in the areas between the magnets, andrings of the same type of non-magnetizable material are located on bothends of the shaft axially adjacent the annulus of permanent magnets andspacers. To prevent the magnets from moving radially outward during thehigh-speed rotation of the rotor, an outer sleeve also made ofnon-magnetizable material is shrink fitted over the magnets, spacers,and rings.

Such construction, while producing acceptable generators for mostaerospace applications, presents several major problems resulting inreduced performance, a high reject rate, and a relatively high unitcost. The most critical performance problem is that of heat buildup inthe rotor. Even if the spacers between the magnets and the ringsadjacent the ends of the magnets are machined very carefully, the heattransfer characteristics of the rotor are not uniform, thus leading toheat buildup which may possibly result in damage to the rare earthpermanent magnets. A second, and closely related, problem is that ofmachining the spacers and rings to fit properly. Since the rotor must bestable at very high rotational speeds, if the rings and spacers do notfit exactly, thus allowing relative movement of the components under theouter sleeve, the result will be an unbalanced condition in the rotorpossibly resulting in dynamic failure of the device.

A further problem encountered in the construction of such a permanentmagnet rotor is that the rotor assembly does not have good rigidity orshaft stiffness, which will in turn reduce the flexure critical speed ofthe device, the maximum speed of rotation without significant dynamicvibration occurring. Since the rotor must be turned at a very high rateof speed, the lack of proper stiffness in the shaft construction willresult in a high rejection rate at best, and possibly in a product whichwill not perform within the required specifications. Finally,construction of the permanent magnet rotor with a shaft, magnets,spacers and rings, and the outer sleeve is an extremely expensive methodof manufacture. The very precise tolerance requirements of the spacersand rings and the high unit rejection rate both add further to the highcost of construction of such rotors.

In certain aerospace applications, it is desirable to have more than oneset of magnets located on a single shaft. In such cases, the shaft isrelatively long and contains a plurality of sets of permanent magnetsspaced axially away from each other on the shaft. For example, a shafthaving three sets of magnets would have sequentially located on theshaft a ring, a set of magnets and spacers, a second ring, a second setof magnets and spacers, a third ring, a third set of magnets andspacers, and a fourth ring. Such an assembly does not have sufficientshaft stiffness to allow the shaft to be turned at the required highspeed. Long before the shaft reaches the desired operation speed, theflexure critical speed will be reached and dynamic failure will occur.

Therefore, it can be seen that a new type of construction for suchhigh-speed, permanent magnet rotor machines is required. Any newconstruction technique must minimize heat buildup in the rotor,eliminate the problem of improper fit between the spacers, magnets, andrings, and ensure that the shaft has sufficient rigidity whilemaintaining or improving the cost characteristic of the device.

SUMMARY OF THE INVENTION

The present invention eliminates the spacers and rings surrounding thepermanent magnets by casting a non-magnetic material, preferablyaluminum, directly on to the steel shaft. Pockets or apertures for thepermanent magnets are cast into the aluminum, and need not be machinedout.

The rare earth permanent magnets are then installed into the pockets onthe shaft, and the rotor assembly may then be machined. Finally, anon-magnetic outer sleeve is installed on the shaft, preferably byheat-shrinking the sleeve onto the shaft. Final balancing of the rotormay then occur.

If the casting operation for the particular rotor assembly to beconstructed is to be done on a relatively small scale, blocks the sizeof the magnets to be used may be installed on flats machined into theshaft, after which the aluminum casting operation may take place. If,however, a substantial number of rotor assemblies are to bemanufactured, the assembly may be die cast by constructing a die withretractable blocks which may be inserted into the mold adjacent theflats on the shaft, after which the aluminum may be injected around theshaft. The blocks may then be retracted from the mold, and the workpiecemay be removed from the die.

Since the aluminum is injected around blocks which are the same size asthe permanent magnets, when the rotor is assembled the aluminum fits themagnets so closely that excess heat buildup in the magnets is no longera problem, since the close-fitting aluminum acts as a uniform and highlyefficient heat sink. Since rings and spacers are no longer used, thetolerance problems accompanying their use is no longer present.Substantial costs are saved in the reduced amount of machining of therotor (and in the elimination of machining spacers and rings) which mustbe done.

Since the aluminum is injected around the entire area of the shaft notto be occupied by the magnets, the completed assembly will haveexcellent rigidity characteristics, greatly increasing the flexurecritical speed and reducing the possibility of dynamic unbalance in therotor leading to machine failure. Finally, since fewer parts andoperations are required by the technique of the present invention, thecost of manufacturing the rotor is significantly reduced. Even rotorassemblies having multiple rotor sections on a single shaft may beconstructed by this technique without substantial difficulty.

DESCRIPTION OF THE DRAWINGS

These and other advantages of the present invention are best understoodthrough reference to the drawings, in which:

FIG. 1 is a perspective view of a shaft which has been machined inpreparation for the casting operation;

FIG. 2. is a perspective view of the shaft of FIG. 1 with blocks mountedin the locations where permanent magnets are to be installed;

FIG. 3 is a perspective view of the shaft of FIG. 2 with aluminum castonto the steel shaft;

FIG. 4 is a perspective view of the shaft of FIG. 3 with the aluminummachined down to the diameter of the magnets and the blocks removed fromthe shaft;

FIG. 5 is a perspective view of the shaft of FIG. 4 with the permanentmagnets installed and the final machining operation performed;

FIG. 6 shows an exploded, perspective view of the installation of theouter sleeve onto the shaft assembly of FIG. 5;

FIG. 7 shows a shaft containing three separate rotors in a completedstate prior to installation of the outer sleeve.

FIG. 8 is a schematic view of a turboalternator utilizing the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 a shaft 10 which has been machined in preparation for thecasting operation is shown. The shaft 10 is first turned to a largerouter diameter, a portion of which is shown as indicated at 20. Theareas on the shaft axially adjacent to the portion of the shaft on towhich the permanent magnets will be located are machined to a secondsmaller diameter, as indicated at 22 and 24.

The larger diameter 20 portion of the shaft 10 then has a number of flatfaces 30 machined uniformly around the shaft, the number of faces 30being equal to the number of magnets to be installed around theperiphery of the shaft 10. Even at their smallest radius, these faces 30extend outwardly somewhat from the smaller diameter areas 22, 24, thesmaller diameter areas 22, 24 being located below the level of themagnets to avoid flux flow therein. The shaft is constructed of amagnetizable material such as steel.

In the preferred embodiment, which is more suitable for construction ofa limited number of rotors rather than full scale production of rotors,two threaded holes 40 are machined into each face 30 on the shaft 10. Itshould be noted that in large scale production utilizing die castingtechniques, the threaded holes 40 would be unnecessary.

In the next step, depicted in FIG. 2, blocks 50 which are exactlyidentical in size and shape to the permanent magnets to be installedlater are mounted onto the faces 30 of the shaft 10. The blocks 50 maybe made out of cold rolled steel. The preferred material to be cast ontothe shaft 10 is aluminum, although other non-magnetizable materialscould be used.

Each block 50 is installed onto a face 30 on the shaft 10 using twoscrews 52, which are countersunk into the blocks 50. The blocks 50 maybe the same size as the magnets to be installed, since aluminum castingdoes not involve substantial shrinkage of the aluminum as it cools. Sothat the blocks may be easily removed from the shaft 10 after casting,the sides of the blocks 50 may be slightly tapered; for example, thesize of the blocks may be one-half degree out of parallel to allow theblocks to be easily removed after the casting operation. The shaft maythen have the aluminum casting operation performed thereon.

FIG. 3 shows the shaft 10 with a cast aluminum surface 60 having sideareas 62 extending over the surface area of the shaft 10 including thetwo reduced diameter areas 22, 24 (FIG. 1). The diameter of the castaluminum 60 should be slightly greater than the outside diameter of theblocks 50 (FIG. 2) to insure that roughness in the outer surface of thealuminum casting will not affect the finished product after machining.

It has been found that by forming a carbon layer on the shaft 10 beforethe blocks 50 are installed and the aluminum 60 is cast aids in ensuringthat the aluminum seeps onto all outer surfaces of the shaft 10,including the areas adjacent the intersection between the blocks 50 andthe shaft 10. Such a carbon layer may be formed on the shaft 10 byrunning the flame from an acetylene torch briefly across the surface ofthe shaft 10.

The shaft 10 together with the aluminum casting 60 is then machineturned to take the aluminum layer 60 down to a diameter slightly largerthan the desired finished diameter, which slightly larger diameter isthe same diameter as the tops of the blocks 50. In addition, the sideareas 62 of the aluminum casting 60 facing the ends of the shaft 10 aremachined flat.

The blocks 50 are then removed from the shaft 10 by removing the exposedscrews 52 holding them in place. In order to aid in the removal of theblocks 50 it has been found that threads of a larger diameter than thatof the screws 52 may be machined into th holes in the block 50 throughwhich the screws 52 fit freely. While the screws 52 fit freely throughthe threaded holes in the blocks 50, a larger diameter screw (not shown)may be inserted into these threaded holes in the blocks 50 after thescrews 52 are removed, and the blocks may be pulled out of the aluminumcasting 60. The shaft 10 and aluminum cast material 60 then appear asshown in FIG. 4.

The next step is to install the permanent magnets, which are typicallyrare earth permanent magnets such as samarium cobalt or al-nickel. Themagnets 80, which are usually of a rectangular configuration having flatfaces, are installed into the apertures 70 (FIGS. 4 and 5), into whichthe magnets 80 fit exactly. Generally, the magnets 80 are so strong thatthey may be installed into the apertures 70 as shown in FIG. 5 withoutthe use of any adhesive since the magnets 80 will be strongly attractedto the magnetizable material of the shaft 10.

In some cases, if the outer diameter of the rotor is large enough, andif the speed at which the device will operate is high enough, it may bedesirable to utilize an adhesive material which may be installed in theaperture 70 before the installation of the magnets 80.

After the magnets 80 are installed, the shaft 10 may then be turned tomachine the flat outer-surfaces of the magnets 80, which as stated aboveare typically flat and must be machined round, as well as to turn thealuminum casting 60 down to the desired finished diameter.

The final assembly step is to install the outer shell 90 on to the rotorassembly 100, as shown in FIG. 6. The outer shell is typicallynon-magnetic steel such as INCO-718, a material which is both extremelystrong and non-magnetic. Another material which may be used for theshell 90 in certain applications is berillium-copper. In either case,the shell 90 thickness is kept to a minimum to keep the distance betweenthe magnets 80 and a stator 300 of the device (FIG. 8) as small aspossible. Typically, the shell 90 thickness may vary from 0.04 inches to0.28 inches. Factors in determining the required shell thickness includethe diameter of the rotor assembly 100, the speed at which the devicewill operate, and the related centrifugal force operating on the rotorassembly 100.

The fit of the shell 90 on the rotor assembly 100 is a high-interferencefit. The preferred method for installation on the shell 90 is to put therotor assembly 100 in dry ice, to heat the shell 90, and to immediatelyinstall the shell 90 onto the rotor assembly 100. The shell 90 is thenimmediately cooled by directing water onto the shell 90 and the rotorassembly 100, to prevent any possible heat damage to the magnets 80. Therotor assembly 100 and the shell 90 may then be balanced by drillingholes 110 in the side of the aluminum casting portion 60 of the rotorassembly 100.

FIG. 7 shows a rotor assembly 200 containing three separate sets ofmagnets which may be manufactured according to the present invention.The rotor assembly 200 is covered with a shell 190. Such a rotorassembly containing multiple sets of magnets is virtually impossible tomanufacture utilizing the spacer and ring technique, but is quite simpleto manufacture using the teachings of the present invention. Theresulting rotor assembly 200 has sufficient shaft stiffness to ensurethat the flexure critical speed is sufficiently high to attain thedesired operating speed.

As mentioned above, if the rotor assembly 100 is to be manufactured inquantity, the preferred technique is to die cast the aluminum coating 60onto the shaft 10. The die (not shown) is constructed so that it hasretractable blocks, and when the shaft 10 is mounted in the die, theblocks are moved inwardly adjacent the flat surfaces 30 on the shaft 10.The molten aluminum is then injected into the die, and the retractableblocks are removed from the die, which may then be opened to remove theshaft 10 containing the aluminum coating 60. The device is then machinedand finished exactly as described above.

One possible use of a rotor constructed according to the teachings ofthis disclosure is in a turboalternator, that is, a rotor assembly 250driven at high speed by a turbine wheel 260, as illustrated in FIG. 8.The rotor assembly 250 is supported by bearings 270 and 280, and isconnected to and drives the turbine wheel 260. A nozzle 290 directs hotgas onto the turbine wheel 260. The stator 300 is mounted around therotor assembly 250. Of course, many other possible uses for the presentinvention exist, and are too numerous to list here.

A rotor assembly manufactured according to the above teachings issubstantially less susceptible to excess heat buildup in the magnets,thus effectively eliminating failure of the device due to destruction ofthe permanent magnets. Since rings and spacers are eliminated by thecasting of aluminum directly on to the steel shaft, the requirements ofhigh tolerance machining of components is substantially reduced, alongwith the high rejection rate of rotors with components not fittingproperly. The resulting rotor has excellent rigidity, even allowing formultiple sets of magnets on a single shaft. Finally, the manufacturingtechnique taught herein substantially reduces the cost of manufacturingrotor assemblies, while resulting in rotor assemblies having higher andmore uniform quality.

What is claimed is:
 1. A permanent magnet rotor, comprising:a shaft ofmagnetizable material; a layer of non-magnetizable material formedaround the periphery of said shaft, said layer having a first pluralityof apertures therein disposed circumferentially about said rotor witheach of said apertures extending radially inwardly through said layer tosaid shaft; a first plurality of rare earth permanent magnets, each ofsaid magnets fitting into one of said apertures in a manner whereby thesides of each of said magnets are surrounded by said layer, said layerthereby acting as a heat sink for said magnets; a sleeve ofnon-magnetizable material installed over said magnets and the portion ofsaid layer surrounding said magnets.
 2. A permanent magnet rotor asdefined in claim 1, wherein said shaft has a plurality of flat areascircumferentially arranged therein, said flat areas being configured toreceive said permanent magnets, said flat areas extending outward fromadjacent surface areas of said shaft to prevent flux flow in saidadjacent surface areas.
 3. A permanent magnet rotor as defined in claim1, wherein said layer is aluminum and is secured onto said shaft.
 4. Apermanent magnet rotor as defined in claim 1, wherein said permanentmagnets are samarium cobalt.
 5. A permanent magnet rotor as defined inclaim 1, wherein said sleeve is thin to maximize flux flow therethrough,having a thickness of from 0.04 to 0.28 inches.
 6. A permanent magnetrotor as defined in claim 1, wherein said sleeve is manufactured ofINCO-718 steel.
 7. A permanent magnet rotor as defined in claim 1,further comprising:a second plurality of apertures in said layerextending around the circumference of said rotor from the outercircumference of said shaft through said layer, said second plurality ofapertures spaced axially away from said first plurality of apertures;and a second plurality of rare earth permanent magnets fitting in saidsecond plurality of apertures.
 8. A permanent magnet rotor, comprising:ashaft of magnetizable material with a plurality of flat surfaces ofrectangular configuration machined circumferentially around the outersurface of said shaft, said flat surfaces being raised from or levelwith the adjacent surfaces of said shaft; a plurality of permanentmagnets installed on said flat surfaces of said shaft, said magnetsbeing of a rectangular configuration matching the shape of said flatsurfaces, the outer surfaces of said magnets being of a predetermineddiameter from the center of said shaft; a layer of non-magnetizablematerial formed on the outer surface of said shaft excluding said flatsurfaces containing said magnets mounted thereon, said layer surroundingsaid magnets and being of an outer diameter equal to said predetermineddiameter of said outer surfaces of said magnets; and a cylindricalsleeve of non-magnetizable material overlying said magnets to retainsaid magnets on said shaft when said rotor is turned at high speed.
 9. Apermanent magnet rotor as defined in claim 1, wherein said permanentmagnets are al-nickel.